Light-sensitive electric device



G. K. TEAL LIGHT SENSITIVE ELECTRIC DEVICE June 12, 1951 Filed Maron 2o, 194e 3 Sheets-Sheet 1 PHOTOC/DUC TIVE SIL ICON CELL 0 000000 aww/05432 002 TIME SECONDS Arm/wmf Patented June 12, 1951 LIGHT-SENSITIVE ELECTRIC DEVICE Gordon K. Teal, Summit, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application March 20, 1946, Serial No. 655,695

(iCl. 20.1-63) 10 Claims.

This invention relates to light sensitive electric devices and more particularly to photoconductive devices and method of making such devices.

An object of this invention is to provide an improved photoconductive device.

This invention is based upon the discovery that a thin film Aof puriiied silicon deposited on a ceramic support exhibits a photoconductive elect when irradiated with visible or near infrared light. Silicon films including small amounts of titanium as an impurity element also exhibit a photoconductive eiiect. Such films also exhibit a large decrease in resistance when their temperature is increased and such decrease in resistance occurs notonly at high temperatures but at relatively low temperatures.

Other materials which may be used to produce photoconductive iilms are germanium and boron. The elements, boron, germanium and silicon are much alike in respect to their response to light and, therefore, may be considered as a group in connection with the manufacture of photoconductive devices.

In an example of practice of this invention, a small ceramic tube, such as a porcelain tube, is provided with a iilm of very pure silicon, preferably by the' pyrolytic deposition of silicon from a gaseous mixture of silicon tetrachloride and hydrogen as the mixture passes over the surface of a heated ceramic tube in a reaction chamber the walls of which are water cooled. A good deposit is obtained with the temperature oi the ceramic tube between 1000 C. (centigrade) and 1250 C. A typical photoconductive lm oi silicon microns thick may be formed in about one and one-half minutes at a ceramic tube temperature of 1100vu C; The cer-amic tube is heated by passing electric current through a tantalum wire located within the bore of the tube.

in another example of practice, a small amount of titanium is deposited simultaneously with the p silicon by mixing gaseous titanium tetrachloride Ag) phototubes.

deposition of these elements from their gaseous chloride compounds.

Electrical contacts for the aboveedescribed lms may be formed advantageously by applying a Yband of colloidal silver paste over the iilms at the ends Vof small sections of the tubes and firing to a bright yellow heat with an oxygen torch. Electrical conductors may be attached tothese bands in any well-known manner, -as by clamping or soldering.

The invention now will be described more in detail, having reference to the accompanying drawing.

Fig. l is a side view or" a photoconductive ydevice, according to this invention.

Fig. 2 is a cross-section of the device of Fig. 1 looking in the direction of the arrows 2.

Fig. 3 is a modified form of the invention particularly adapted for use as afvariable resistor having a high negative temperature coeicient of resistance. v

Fig. 4 shows graphs of the spectral sensitivity of photoconductive silicon devices, according to this invention, and cesium-oxygenesilver (Csi-O.-

Fig. 5 shows graphs of the dynamic response of photoconductive silicon devices according to this invention and of a Thalode cell.

Fig. `6 shows a graph of the variation of photo response with change of illumination of a silicon device according to this invention.

Fig. 7 shows a graph of the variation `of photo response with change of voltage of a silicon device according to this invention.

Fig. 8 illustrates an apparatus suitable for the pyrolytic deposition of photoconductive lms on ceramic tubes, according to this invention.

Fig. 9 is a modified form of photoconducting device according to this invention.

Fig. 10 shows graphs of the variation of the conductivity of silicon iilms with change of temperature.

The same reference characters are used to designate identical elements in the several iigures of the drawing. v,

Referring now to Figs. l and 2 of the drawing, a photoconductive device E0, according to this invention, comprises a tube Il of porcelain or sillimanite on which a thin lm I2 of highly purified silicon is deposited by pyrolytic chemical reaction of silicon tetrachloride ('SiCli) and hydrogen. Near each end of the film-coated tube H andover the film l2 are contacts `I3 and 14 of red silver paste. A tantalum heater wire t5 is located within and substantially fills the bore of tube II. This wire is formed with loops at each end for a purpose to be described hereinafter. The wire I may be removed after the lm has been deposited and either before or after the contacts I3 and I4 have been applied. It is to be understood that the dimensions of the several elements illustrated in Figs. 1 and 2 are greatly exaggerated for purposes of illustration. The lm I2 and the contacts I3 and I4 are very much thinner with respect to the diameter of the tube II than as illustrated.

For use as a photoconductive device a source of electrical potential, such as a battery I6, a load circuit, such as resistor II, and a current indicator, such as microammeter I8, may be connected in series and by a switch 2| and conductors I9 and 20 to the contacts I3 and I4. Conductors i9 and 29 may be soldered or otherwise held in contact with the contacts I3 and E4, respectively.

v'The device 25, illustrated in Fig. 3, is similar to the device of Figs. 1 and 2 except that it is Yespecially formed to function as a variable resistor having a high negative temperature coei.- cient of resistance. Such a device may be called a thermistor. Device 25 comprises a ceramic tube 26, a thin lm 21 of silicon deposited on the tube 25, terminal contacts 28 and 29 of red silver paste, and a tantalum heater wire 30. The film 21 has a helical slit 3I cut through its surface for the purpose of increasing the resistance between the contacts 28 and 29. As illustrated, the slit 3l consists of only two turns but any number of turns may be used, depending upon the result desired. Electrical conductors may be connected to contacts 28 and 29, as described in connection with conductors I9 and 20 in Fig. 1.

Photoconductive devices according to this invention have highly desirable characteristics, as will now be explained.

In Fig. 4 graph A shows the spectral sensitivity of a photoconductive silicon cell, and graph B shows the spectral sensitivity of a Cs-O-Ag phototube. The absolute magnitude of response of each of these devices is not indicated by graphs A and B since the maximum response of each device is shown equal to 100 on an arbitrary photocurrent scale; however, for each device the responses to light of diii'erent wavelengths are on an equal energy basis. The maximum response for the silicon cell lies between 8400 and 8600 A. U. (Angstrom units). The long wavelength limit has not been determined accurately but lies in the neighborhood of 12,000 to 15,000 A. U. The maximum response of the Cs-O-Ag phototube as shown by graph B is at a considerably shorter wavelength as is also the case with the long wavelength limit of response. The photoconductive silicon cell shows a more nearly equal response over a much greater range of wavelengths than the Cs-O-Ag phototube.

In Fig. 5 graphs C and D illustrate the relative speeds of response respectively of a photoconductive silicon cell and a typical Thalode cell. These graphs represent the patterns obtained on a cathoderay oscillograph while the device is illuminated with an intermittent light signal of about 240 cycles per second. The scales for the different devices are arbitrary. The speed of response of the photoconductive silicon cell is considerably faster than that of the Thalofide cell. The photoconductive silicon cell reaches substantially maximum response in about 0.001 second. This is an important characteristic taken in connection with the fact that such silicon cells may be heated to a red heat in an oxygen gas flame without decrease in sensitivity when cooled to room temperature.

From graph F of Fig. 6 it is seen that the response of a photoconductive silicon cell is approximately proportional to the illumination. This graph shows the response for light of wavelength approximately 9200 A. U. Levy line screens were used as filters to effect the change in illumination, their transmission being measured under the conditions of use.

In Fig. 7 graph G shows the variation in the response of a typical photoconductive silicon cell for variation in voltage impressed upon the terminals of the device. The relative responses were determined by Vusing intermittent illumination and a cathode ray oscillograph. Up to 40 volts the responses are linear as shown by graph G.

- Apparatus for depositing films of silicon, or silicon mixed with titanium, on the surfaces of ceramic tubes will now be described with reference to Fig. 8. The ceramic tube II provided with tantalum wire I5, as described hereinbefore in connection with Figs. l and 3, is suspended in the reaction chamber 40. In order to introduce the ceramic tube into the reaction chamber 40 the head 4I is unscrewed and removed therefrom, the water circulating hoses 42 and 43 being first detached. Molded or otherwise fixed in the head 4l are terminal and cooling fixtures for suspending the ceramic tube II and tantalum wire I5. These xtures comprise a large U-shaped tube 44 anda'smaller U-shaped tube 45. These tubes are preferably of copper and they serve to conduct cooling liquid through the heat-distributing terminals 46 and 4'I between which tantalum wire I5 is suspended by retractile springs 48 and 49. After the tantalum Wire I5, carrying the ceramic tube I I, has been suspended between the terminals 46 and 4l the head 4I is screwed back into place closing the open end of the chamber 40. The'circulating hoses 4'2 and 43, the purpose of which will be explained hereinafter, are again connected asv shown in Fig. 8.

The apparatus for administering the desired gaseous mixture, such as a mixture of hydrogen and silicon tetrachloride, to the reaction chamber 40 is also shown in Fig. 8. The hydrogen is obtained from a supply tank 50 under control of valve 5I from which the hydrogen gas enters the deoxidizing furnace 52 for the purpose of removing any trace of free oxygen that may be mixed with the hydrogen. From the furnace 52 the hydrogen and any water vapor that may be formed in the furnace enters the drying towers 53 and 54. These drying towers may be provided with phosphorus pentoxide for removing al1 traces of water vapor, thus permitting only pure hydrogen to flow into the outlet pipe 55.

A supply of dry nitrogen for flushing the system may be obtained from a nitrogen supply tank 55 under control of valve 5? from which the nitrogen gas enters a deoxidizing furnace 58 for removing traces of free oxygen that may be mixed with the nitrogen. From the furnace 58 the nitrogen and any water vapor that may be formed in the furnace 58 enters the drying towers 59 and 65. These towers may be of the same construction as towers 53 and 54 permitting only pure nitrogen to flow through fiowmeter 6I when valve 62 is opened to permit nitrogen to flow into the system through tube 63.

Silicon tetrachloride in gaseous form is derived from boiler 64 where it is rst Vmixed with hydrogen gasl under conditions that give the mixture a high degree of saturation. The mix- .ensuciarv T ture is effected by introducing pure hydrogen gas through the inlet tube B5 into the vapor of silicon tetrachloride in the upper portion of boiler 64 under control of the valve 66 after the hydrogen passes through the flowmeter 61 from the outlet tube 55. The vapor is produced by gently boiling liquid silicon tetrachloride in boiler 64 by heat generated in electric heater |05. The hydrogen gas after mixing with the silicon tetrachloride vapor in the boiler.' B4, passes through the boiler outlet pipe 68 into the redux condenser 69 which is packed withglass helices. From the condenser 6B the gaseous mixture of hydrogen and silicon tetrachloride vapor passes through valve 10 and'tube 'll to the intake tube 12 of the reaction chamber 130. i A by-pass tube 'I3 with valve 'I4 is also provided for diluting the gaseous mixture, if desired, by passing some of the hydrogen directly into the reaction chamber 40.

The water for the cooling system is obtained from a source 76 of cool Water and raised to a predetermined temperature, preferably. a few degrees above freezing, by means of a heater 11. The cooling water passes through a thermometer chamber i8 which gives a continuous indication of the temperature of the water and enters the reflux condenser E9 to hx the concentration of the gaseous mixture from the boiler 64. From the condenser 69 the cooling water passes through a second heater '19 and Ithermometer chamber 8|] into the cooling jacket 3l surrounding the walls of the reaction chamber 45 by way of hose 43, U-,shaped tube e4, hose 82, U-shaped tube 45 and hose 42. The cooling water passes. from the cooling jacket 8l through waste t'ubell).

In order to provide for the simultaneous deposition of titanium, 'if desired, an arrangement for adding a mixture of hydrogen and titanium tetrachloride is provided. Titanium tetrachloride in a gaseous form is Aderived from boiler 83 where it is mixed with hydrogen gas under conditions which give the mixture a high, degree of saturation. This mixture is effected by introducing pure hydrogen gas through inlet tube 84 into the vapor of titanium tetrachloride in the upper portion of boiler 83 under vcontrol of valve 85 after the hydrogen passes through flowmeter 8B from the outlet tube 55. The Vapor is produced by gently boiling liquid titanium tetrachloride in boiler 83 by heat generated in electric heater IUS. The hydrogen gas, after mixing with the titanium tetrachloride vapor, passes through the boiler outlet tube 3l into `the reflux condenser 88. From the condenser 88 the gaseous mixture of hydrogen and titanium tetrachloride vapor passes through valve 89 and tube '90 vto the reaction chamber 40 through intake tube l2.

Cooling 'Water for reflux condenser 88 is lobtained from source 9| of cool water heated 'in heater 92, passed through a thermometer chamber 93 into the reflux condenser 88 and out through waste tube 94.

The necessary heat for producing the chemical reaction of the gaseous mixture within the reaction chamber 40 is derived from a source of electrical current 95. .A suitable regulator 9B serves to maintain the voltage at a value suitable to produce the required temperature Within the chamber '40. The energy from source 'S5 is supplied to the suspended ltantalum heater wire within the chamber lil by connecting the secondary winding of'transformer to the circuiating tubesf44 and 15, thus 'causing a current to new through the wirev I5 -by Wayof-the-terminals 46 and 41 and the suspension springs 48 and 49 when primary switch 98 is closed.

The flowmeters El, 6l, 'I5 and 8B comprise tapered glasstubes in which glass balls or hard rubber floats ioat as the gases now through from the bottom upward. The position of the iloating element gives an indication of the rate of 110W.

' If boron or germanium is to be deposited, the boiler B4 may be used to provide a mixture of hydrogen with vapors of a compound of the desired one of these elements. Nitrogen from source 5e may be used to ush the system whenever desired.

When it is desired to deposit a film of pure silicon on the ceramic tube Il silicon tetrachloride is used, preferably in boiler S4. Boiler 83 is not used and valves 85 and 89 are closed. Dry tank hydrogen is passed over the ceramic tube Il for several minutes by opening valves 5I and 14 While all of the other Valves in the system are kept closed. Hydrogen from Athe tank 5B passes through the Valve 5l, furnace 52, drying towers 53 and 511, outlet tube 55, owmeter l5, valve 14, by-pass tube 13, inlet tube 'F2 to the reaction chamber 0 and out through the waste tube 99. While the hydrogen is ovving through the reaction chamber 40 the switch 98 is closed and tantalum Wire l5 is raised toa temperature sufficient to heat the ceramic tube Il to l200 C. at which temperature it is maintained for about 30 seconds. The temperature of the tube H is determined by any Well-known means, such as an optical pyrometer. The tube Il is now ready to be coated with pure silicon, the thickness and character of the lm being determined by the hydrogen iiow, the silicon tetrachloride iiow, the time of deposition, the temperature Vof the ceramic tube and the type or" ceramic used for the tube. By suitably controlling these variables a` coherent, smooth, grey to black nlm may be obtained on ceramics.

Preparatory to effecting the deposition the valve 10 is opened and the openings of valves 14 and 66 are adjusted to give the desired ratio between the amount of hydrogen flowing through the bypass tube 'I3 and the amount flowing through tube into the silicon tetrachloridel boiler 64. This ratio may be ofthe order of 1:1, as determined by the observation of owmeters l5 and $1. The heater 'l'l is adjusted to a tempera/ture of about 3 C. and may be thermostatically controlled. The heater 19 is adjusted to maintain the water entering the cooling system of the reaction chamber 40 at approximately room teperautre, say 25 C.

After several minutes of iiow to establish the equilibrium of the gaseous mixture of hydrogen and silicon tetrachloride the temperature of the ceramic tube Il is raised to approximately 1100" C. or any other desired tempera-ture between about 1000o C. and 12.50 C. and maintained `for the period of the deposition which maybe one and one-half minutes for a temperature of 11'00" C. .Dur-ing this period chemical reaction Yoccurs between the gaseous silicon tetrachloride andthe hydrogen, the liberated pure silicon is deposited on the surface of the ceramic tube H and the unwanted products of the reaction are discharged through the Waste tube 99.

If it is assumed that the hydrogen gas emerging from the boiler 64 .is highly saturated, possibly supersaturated, with silicon tetrachloride, the :condensation occurring 'within the condenser 69 will reduce 'the concentration of the silicon 7. tetrachloride to the saturation value corresponding to the temperature within the condenser. Therefore, when the mixture enters the reaction chamber 40 it encounters a chamber Wall temperature which is somewhat higher than that of the condenser. This diierential increases the saturation pressure of the gaseous mixture to a point which is safely above the partial pressure of the silicon tetrachloride and thereby prevents condensation of the silicon tetrachloride on the walls of the reaction chamber. The same differential temperature relation is maintained between the condenser and the terminal xtures within the reaction chamber 40 by means of the cooling tubes M and 45 and the external tubes 42, d3 and 82 through which the cooling water circulates.

The coated ceramic tube H may be removed from the reaction chamber 40 and provided with terminal contacts, as described hereinbefore.

If the deposited lm is to contain titanium, hydrogen saturated with titanium tetrachloride is mixed with the hydrogen and silicon tetrachloride mixture described above by simultaneously opening valves 85 and 89 to obtain a desired ow of hydrogen saturated with titanium tetrachloride through outlet pipe 90. The heater 92 maintains the cooling water in reflux condenser 88 at approximately 2 C.

Boron and germanium may be deposited instead of silicon by replacing the silicon compounds in boiler 64 with a desired compound of boron or germanium.

The modified photoconductive device of Fig. 9 comprises a glass plate 32 on one face of which a layer 33 of silver is vaporized. A relatively thick layer 34 of highly puried silicon isvaporized on the layer of silver. On the surface of the silicon layer is a light transmitting layer 35 of gold formed by vaporization. Around the outside surface area of the layer 35 is deposited a ring 36 of metal, such as gold or copper, which layer is relatively thick and to which conductors may be attached. An electrical circuit including a source of current,l such as battery 31, a load device, such as resistor 38, and a current indicating device, such as milliammeter 39, may be connected in series between the silver layer 33 and the metal ring 3e which serve as terminals for the photoconductive device. An increase in specific conductivity occurs when the layer 34 of silicon is illuminated by light passing through the layer 35 of gold, as indicated by the arrows in Fig. 9.

Fig. 10 shows plots of the logarithm of the specific conductivity of silicon iilms versus the reciprocal of the absolute temperature. The graphs, H, I, J, K, L, M and N show data obtained on various silicon films deposited on porcelain in the manner hereinbefore described. These graphs show three distinctly different regions; a straight line high temperature region represented by the left-hand portion of graph O which is a representative graph for this temperature region for all the data Iof graphs H to N,

a straight intermediate temperatureregion andY a `curved low temperature region. The curvature in the low temperature region is the reverse of that obtained with fused silicon samples of the prior art. Such curvature extends to temperatures at least as low as 180 C. for graph K, although that graph is not extended so far in Fig. 410. There are two inflection points: one at about 2,27.C. and another at 400 C. or higher, thesxactitemrerature in the latter case depending on the magnitude of the conductivity of thea-Ae 2M, (l)

where k is Boltzmanns constant, T is the absolute temperature, fr is the conductivity per centimeter, E is the energy separation of the iilled and unfilled bands, and A is a factor that varies slowly with temperature. The factor A may therefore be written ANT) as disclosed on page 191 of the book by F. Seitz entitled The Modern Theory of Solids published by McGraw-Hill Book Company, incorporated in 1940. In the case of graph K the value of activation energy is approximately Zero and slightly negative in value indicating that the impurity level probably lies close to or within one of the bands of the solid.

The values of activation energy for the intermediate region vary, individual values of 0.31, 0.45, 0.41, 0.49 and 0.78 electron volts being obtained from the data on which graphs H to M of Fig. 10 are based.

In the high temperature region individual f values of E obtained are 1.13 e. v. (electron volts) 1.106 e. v. and 1.12 e. v. The straight line graph O, as above stated, is the best t that can be obtained Yfor all the high temperature measurements. It represents a band separation in silicon of 1.12 electron volts. The high temperature data leads to a calculated value of 3.9 106 ohm-1 cm.1 for the specific conductivity of pure silicon at 25 C. The lowest specic conductivity observed was 1.8 10-5 ohrrrl cmrl. For this sample the activation energy at room temperature was the highest found, equal to 0.97 electron volt.

The best photoconducting silicon cells are the ones showing the lowest specic conductivity corresponding to very pure silicon. The long wavelength of the photoconductive effect has not been measured accurately, but appears to lie between 12,000 and 15,000 A. U. The thermal activation energy of 1.12 e. v. corresponds to a wavelength )\o=11,000 A. U. This is considered strong evidence for the same electron bands being concerned in both the photoelectric and thermal processes of increasing the conductance. The maximum photoelectric sensitivity occurs at 8,400 to 8,600 A. U. which is equivalent to 1.45 electron volts energy. It seems certain that the process is not simply a transition between an impurity level and adjacent bands, since this would result in a much lower value of to than was observed.

Observations on the optical transparency of silicon indicated a region of rapidly increasing transparency at 8750 A. U. and extending into the infrared. The high absorption of silicon in the visible and near infrared regions shows a tail extending to about 10,500 A. U. This corresponds to a separation of the conducting and non-conducting bands of silicon equivalent to 1.18 electron volts. Evaporated films also show photoconductance. Maximum photoelectric sensitivity of pyrolytic ilms is obtained at about the same wavelengthas that at which the absorption becomes appreciable.

The photoconductance effect is also present in silicon films in which titanium has been depos- 9 ited, but due to the higher conductivity of the material a change in conductivity due to light is not as great as in the lms of pure silicon.

What is claimed is:

1. A method of producing a photoconductive layer which comprises supporting a body of ceramic material in a reaction chamber, administering a gaseous mixture consisting of substantially pure hydrogen and the vapor of a chloride compound of one of the elements of the group of elements consisting of silicon, boron and germanium to said chamber, and heating said ceramic material to a temperature of approximately 1100" C. to effect the chemical reaction of the chloride compound of said gaseous mixture and the hydrogen and the deposition of a layer of the said element of said compound of low specific conductivity on the exposed surface of said ceramic material, the specific resistance of which layer changes a useful amount upon change of irradiation thereof with light of wavelength within the range of wavelengths of visible and near infrared light.

2. The method of producing a photoconductive layer of purified silicon which comprises supporting a body of ceramic material in a reaction chamber, administering a gaseous mixture consisting of silicon tetrachloride and substantially pure hydrogen to said chamber, and heating said ceramic material to a temperature of approximately 1100" C. for a period of approximately one and one-half minutes to effect the chemical reaction of said silicon tetrachloride and hydrogen and the deposition of a layer of silicon of low specific conductivity on the exposed surface of said ceramic material, the specic resistance of which layer of silicon changes a useful amount upon change of irradiation thereof with light of wavelength within the range of wavelengths of visible and near infrared light.

3. A photoconductive device comprising a plate of glass, a layer of silver vaporized on one surface of said plate, a layer of silicon vaporized on said layer of silver, and a light transmitting layer of gold vaporized on said layer of silicon, said layer of silicon being adapted to have its specific resistance as measured between said layers of silver and gold changed by irradiation with light transmitted through said layer of gold.

4. A photoconductive device comprising a ceramic support, a thin film of material on said support the specific resistance of which changes a useful amount upon a change of irradiation thereof with light of wavelength within the range f wavelengths of visible and near infrared light, said material consisting of an element in substantially pure state from the group of elements consisting of silicon, boron and germanium, and electrical conductors contacting separated portions of said film.

5. A photoconductive device comprising a ceramic support, a thin film of highly pure silicon on said support the specific resistance of which changes a useful amount upon a change of irradiation thereof with light of wavelength within the range of wavelengths of visible and near infrared light, and electrical contacts contacting separated portions of said film.

6. A photoconductive device comprising a ceramic support, a Vthin film of highly pure boron on said support the specic resistance of which changes a useful amount upon a change of irra- 10 diation thereof with light of wavelength within the range of wavelengths of visible and near infrared light, and electrical conductors contacting separated portions of said film.

7. A photoconductive device comprising a ceramic support, a thin film of highly pure germanium on said support the specific resistance of which changes a useful amount upon a change of irradiation thereof with light of wavelength within the range of wavelengths of visible and near infrared light, and electrical conductors contacting separated portions of said film.

8. A photoconductive device comprising a ceramic tube, a thin film of highly pure silicon on said tube the specific resistance of which changes a useful amount upon a change of irradiation thereof with light of wavelength within the range of wavelengths of visible and near infrared light, and separated bands of fired silver paste surrounding and on top of said lm near the ends of said tube.

9. A photoconductive device comprising a, ceramic support, a lm of highly pure silicon on said support having a thickness of approximately 5 microns the specific resistance of which changes a useful amount upon a change of irradiation thereof with light of wavelength within the range of wavelengths of visible and near infrared light, and lelectrical contacts contacting separated portions of said film.

10. A photoconductive device comprising a ceramic tube, a lm of highly pure silicon surrounding the convex surface of said tube formed by supporting said tube in a reaction chamber, administeringy a gaseous mixture consisting of silicon tetrachloride and substantially lpure hydrogen to said chamber and heating said tube to a temperature of approximately 1100 C. for a period of approximately one and one-half minutes to effect the chemical reaction of said silicon tetrachloride and hydrogen and the deposition of a layer of silicon of low specific conductivity on the convex surface of said tube, the specic resistance of which layer of silicon at room temperatures changes a useful amount upon changes of irradiation thereof with light of wavelength within the range of wavelengths of visible and near infrared light, and separated bands of fired silver paste surrounding and on top of said film.

GORDON K. TEAL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 718,891 Acheson Jan. 20, 1903 866,462 Hammer Sept. 17, 1907 876,331 Clark Jan. 14, 1908 1,019,391 Weintraub Mar. 5, 1912 1,039,463 Thomson Sept. 24, 1912 1,349,053 Feild Aug. 10, 1920 1,739,256 Pender Dec. 10, 1929 1,964,322 Hyde June 26, 1934 2,281,843 Jira May 5, 1942 2,313,410 Walther Mar. 9, 1943 2,423,476 Billings July 8, 1947 OTHER REFERENCES Bidwell, Physical Review, May 1922, pages 447-455. 

1. A METHOD OF PRODUCING A PHOTOCONDUCTIVE LAYER WHICH COMPRISES SUPPORTING A BODY OF CERAMIC MATERIAL IN A REACTION CHAMBER, ADMINISLTERING A GASEOUS MIXTURE CONSISTING OF SUBSTANTIALLY PURE HYDROGEN AND THE VAPOR OF A CHLORIDE COMPOUND OF ONE OF THE ELEMENTS OF THE GROUP OF ELEMENTS CONSISTING OF SILICON, BORON AND GERMANIUM TO SAID CHAMBER, AND HEATING SAID CERAMIC MATERIAL TO A TEMPERATURE OF APPROXIMATELY 1100* C. TO EFFECT THE CHEMICAL REACTION OF THE CHLORIC COMPOUND OF SAID GASEOUS MIXTURE AND THE HYDROGEN AND THE DEPOSITION OF A LAYER OF THE SAID ELEMENT OF SAID COMPOUND OF LOW SPECIFIC CONDUCTIVITY ON THE EXPOSED SURFACE FO SAID CERAMIC MATERIALS, THE SPECIFIC RESISTANCE OF WHICH LAYER CHANGES A USEFUL AMOUNT UPON CHANGE OF IRRADIATION THEREOF WITH LIGHT OF WAVELENGTH WITHIN THE RANGE OF WAVELENGTHS OF VISIBLE AND NEAR INFRARED LIGHT. 